Strain gauge assembly, particularly for an axle counter

ABSTRACT

A method for assembling a strain gauge arrangement for an axle counter, includes: a strain sensor element; a carrier, to which the strain sensor element is fastened; and a railroad track structure to be monitored. When the carrier is fastened to the structure at least a part of it is kept in an elastically deformed state. The carrier is embodied with a first carrier piece opposing a second carrier piece. The strain sensor element is fastened to the first carrier piece with a first fixing point, to the second carrier piece with a second fixing point, and to neither the first nor the second carrier piece with a central section between the fixing points. At least the part of the carrier is elastically braced by means of a bracing element for when adhesively bonding to the structure to be monitored. Thereafter the bracing element is removed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This continuation application claims priority to continuationapplication Ser. No. 16/826,220 filed on Mar. 21, 2020; which itselfclaims priority to PCT/EP2018/073820 filed on Sep. 5, 2018, which haspublished as WO 2019/057491 A1; and also European application number17192644.7 filed Sep. 22, 2017; the entire contents of which are fullyincorporated herein with these references.

DESCRIPTION Field of the Invention

The invention relates to a strain gauge arrangement. In particular, theinvention relates to an axle counter comprising at least one strainsensor element, in particular an optical fiber with a fiber Bragggrating, a carrier, to which the strain sensor element is fastened, anda structure to be monitored, in particular a railroad track, to whichthe carrier has been fastened, wherein at least a part of the carrier iskept in an elastically deformed state by the structure when the carrieris fastened to the structure.

Background of the Invention

Such a strain gauge arrangement for an axle counter has been disclosedby EP 3 069 952 A1, in particular.

Axle counters are used to make rail traffic safer. Axle counters can beused to check, in particular, whether the location of the axle counterhas been completely passed by a train, for example to determine whetherassociated track sections have become completely free.

A possible measuring principle of axle counters is based on using astrain sensor element to measure an elastic deformation at a rail causedby the weight of a train transferred by an axle.

In this respect, EP 3 069 952 A1 proposes to fasten a fiber Bragggrating (FBG), as a strain sensor element, to a rail under pretension.The reflection wavelength of the fiber Bragg grating depends on theelastic strain state of the FBG, on the basis of which the strain statecan be measured. As a result of the pretension, the strain state of theFBG can be used to identify whether the FBG is still correctly fastenedto the rail or has fallen off the rail. One variant proposes thermalapplication of the pretension by virtue of fastening a carrier to therail under pretension.

Moreover, DE 10 2005 010 344 A1 has disclosed the practice ofinfluencing damping properties in an optical waveguide via bendingstates.

Furthermore, strain sensor elements which are based on a change inresistance or capacitance due to a changing strain state, for example,have also been disclosed.

However, it is comparatively difficult to fasten strain sensor elementsunder pretension directly on a structure to be monitored, for instance arailroad track. A certain simplification can be achieved by applying thepretension via a carrier to which the strain sensor is affixed. However,it is also difficult in this case to set a defined strain state on thestrain sensor element (for instance, to set a working point).

SUMMARY OF THE INVENTION Object of the Invention

The invention is based on the object of presenting an easy-to-use straingauge arrangement in which a fall-off can be reliably recognized and astrain state of the strain sensor element can be specified more easily.

BRIEF DESCRIPTION OF THE INVENTION

This object is achieved by a strain gauge arrangement of the type setforth at the outset, which is characterized in that the carrier isembodied with a first carrier piece and a second carrier piece, whichoppose one another in spaced apart fashion, and in that the strainsensor element is fastened to the first carrier piece with at least onefirst fixing point, to the second carrier piece with at least one secondfixing point, and to neither the first nor the second carrier piece witha central section between the fixing points.

Within the scope of the invention, provision is made for the strainsensor element not to be fastened to the carrier in linear fashion overits entire measuring length, but rather only to be fastened to a firstcarrier piece with a first fixing point and to a second carrier piecewith a second fixing point. The strain sensor element is kept free inthe central section situated therebetween.

As a result, it is possible, as a matter of principle, to at leastpartially decouple the strain state of the strain sensor element fromthe elastic deformation state of the carrier. In particular, it becomespossible to transfer an elastic deformation of the carrier resultingfrom a deformation of the monitored structure in a weakened orreinforced fashion, depending on the application, or to transfer saidelastic deformation to the strain sensor element only above certainthresholds, or to transfer certain types of deformation of the carrierto the strain sensor element in reinforced fashion. As a result, thestrain sensor element can be operated in an optimal working range, andcertain monitoring functions of the strain sensor element, for instancethe detection of a fall-off from the structure, can be made morereliable. At the same time, the carrier can be fastened to the structurewith an easily controllable elastic deformation of the carrier, withoutbeing excessively restricted by the strain sensor element.

A simple and direct access to the strain sensor element is possible atthe central section, in particular without being fixed in thedeformation of the strain sensor element by preforming an underlyingcarrier. In the central section there can be, e.g., additionaltightening (for instance by means of a driver element) or an additionalrelief (for instance by excess length) of the strain sensor element withrespect to the carrier.

Fastening the carrier to the structure to be monitored, with at least apart of the carrier being kept in an elastically deformed state by thestructure, enables a simple check to be made as to whether the straingauge arrangement is still correctly seated on the structure or hasfallen off the structure; the latter leads to springing back into anelastically non-deformed state of the carrier or at least of said part.By way of example, falling-off from the structure may occur with agingor temperature-related softening of a fastening. The strain gaugearrangement is preferably embodied in such a way that the elasticdeformation or bracing of the carrier is ascertainable via a measuredstrain of the strain sensor element (i.e., the elastic deformation ofthe carrier lies in the monitored region of the strain sensor element).Alternatively, it is also possible to provide a measuring device ormeasurement which is separate from the strain sensor element or thestrain thereof and by means of which the deformation of the carrier canbe checked (for instance using a further strain sensor element).

A strain gauge arrangement usually comprises only one strain sensorelement; alternatively, it may comprise also two or even more strainsensor elements.

Within the scope of the invention, a strain sensor element is fastenedto a first and second carrier piece with a first and second fixing pointand not fastened to these carrier pieces (or to other parts of thecarrier either) in a central section. In addition to the first andsecond carrier piece, the carrier usually also comprises one or morefurther pieces (for example a base element or a bracket).

The strain sensor element is preferably embodied as an optical fiberwith a fiber Bragg grating (FBG); the FBG is located between the fixingpoints in the central section in this case. Typically, the strain sensorelement is under a slight elastic strain when the carrier is fastened tothe structure, and so both compression and extension of the structure tobe monitored can easily be identified.

A strain gauge arrangement according to the invention can be used in anaxle counter, in particular; such an axle counter comprises at least onestrain gauge arrangement according to the invention. However, otherapplications for the strain gauge arrangement can also be envisaged, forinstance for monitoring mechanical loads or wear on machine components.A strain sensor element can be disposed at an oblique angle, forinstance at an angle of approximately 45°, with respect to a neutralfiber of a structure to be monitored. By way of example, the carrier canbe attached to the structure to be monitored by screwing, soldering oradhesive bonding. By way of example, the strain sensor element can befastened to the carrier pieces by jamming, soldering or adhesivebonding.

In summary, the invention particularly easily allows, firstly, thestrain sensor element (when the carrier is fastened to the structure) tobe kept at a desired pretension, and, secondly, the monitoring by meansof at least the part of the carrier as to whether the carrier is still(sufficiently) fastened to the structure to be monitored.

Preferred Embodiments of the Invention

In a preferred embodiment of the strain gauge arrangement according tothe invention, provision is made for the part of the carrier which iskept in an elastically deformed state by the structure when the carrieris fastened to the structure to comprise a leaf spring element, and forthe following to be formed on the leaf spring element: a support elementfor support on the structure when the carrier is fastened to thestructure and a driver element for reaching behind the strain sensorelement in the central section. An elastic deformation is impressed onthe leaf spring element by the support element abutting against thestructure. The driver element typically moves substantiallyperpendicular to the direction of extension (measurement direction) ofthe strain sensor element and typically moves substantiallyperpendicular to the surface of the structure to which the carrier hasbeen fastened. The driver element can be used to impress an elasticstrain or a bend on the strain sensor element, depending on thedeformation of the leaf spring element. This strain or bend can bemeasured by the strain sensor element (for instance via the strain assuch, or via a modified, usually increased damping), and the detachmentof the carrier from the structure can thus be easily identified. Thisdesign particularly reliably detects a detachment of the strain sensorelement from the structure to be monitored. All that is needed to detectthe sensor falling off is an elastic deformation of the leaf springelement; in particular, there is no need to fasten (for example,adhesively bond) carrier surfaces that are elastically braced againstone another to the structure, which makes installation particularlyeasy.

In a preferred development of this embodiment, the leaf spring elementand the driver element are embodied in such a way that no strain, oronly little strain, is applied to the strain sensor element by said leafspring and driver elements in the elastically deformed state of the leafspring element when the carrier is fastened to the structure, and agreater strain is applied to the strain sensor element in an elasticallyrelaxed state of the leaf spring element after the carrier has beendetached from the structure. In its elastically relaxed state, the leafspring element (together with the driver element) tightens and/or bendsthe strain sensor element, which is usually easy to detect; conversely,the leaf spring element and the driver element (and the support element)are embodied in such a way that, in the elastically tensioned state, thestrain sensor element is not or only slightly tightened by the driverelement. Alternatively, it is also possible to relax the strain sensorelement in the elastically relaxed state of the leaf spring element.

An embodiment in which the part of the carrier which is kept in anelastically deformed state by the structure when the carrier is fastenedto the structure comprises the first carrier piece and the secondcarrier piece, wherein the first carrier piece and the second carrierpiece are elastically braced with respect to one another with at least adirection component along an extension direction of the strain sensorelement, is also advantageous. This design is particularly simple. Thedirectional component of the elastic deformation of the carrier actsdirectly on the strain state of the strain sensor element.

A development of this embodiment in which the first and second carrierpiece are connected to one another by a bracket is preferred, inparticular wherein the bracket is spaced apart from the strain sensorelement by a distance AS, preferably where the following applies to thedistance AS and a distance AE from the first to the second fixing point:AS≥1*AE, particularly preferably AS≥3*AE. The bracket (sometimes calleda bar) acts as a fixed joint; around this, the carrier pieces can betwisted elastically against one another. The force on the strain sensorelement can be set by means of suitable distances (and a suitableelastic deflection); with AS≥3*AE, this is possible in particularlyaccurate fashion. If desired, one or more springs can be disposedbetween the carrier pieces in order to modify the elastic behavior ofthe carrier. As an alternative or in addition thereto, provision can bemade of a connection of the carrier pieces via a base element.

A development in which the carrier forms a bracing bearing, at or inwhich a bracing element, which engages on the first and second carrierpiece, is arrangeable, is preferred, in particular where the bracingbearing and bracing element interact in the style of a cam. With thebracing bearing and the bracing element, an elastic deformation can betemporarily impressed on the carrier or at least the part thereof, inparticular while the carrier is being fastened (for instance adhesivelybonded) to the structure. The application of force is particularlysimple, in particular also in manual fashion, using a cam.

Advantageously, a bracing element arranged at or in the bracing bearinghas been broken off from the carrier, in particular from the innerregion of the bracing bearing. The bracing element can initially bemanufactured as part of the carrier, and is then directly availableduring installation together with the carrier. For use in the bracingbearing, the bracing element (e.g. a cam) can be broken off from thecarrier (for instance broken out of the bracing bearing), and used forbracing in the bracing bearing (if necessary) and activated, inparticular rotated. Alternatively, the bracing element can also lose apreviously existing tensioning effect by breaking off.

It is also advantageous if no bracing element is arranged at or in thebracing bearing any more. During normal operation (i.e., when measuringthe strain on the structure), the tensioning element then no longerexerts a force on the carrier, and so the elastic state thereupon onlydepends on the fastening to the structure, as a result of which thestrain gauge arrangement falling off can easily be identified.

An embodiment which provides for the carrier to have a base element, bymeans of which the carrier is fastened to the structure to be monitored,and for the first carrier piece and the second carrier piece to befastened on or at the base element is preferred. Targeted and simplifiedfastening to the structure, in particular also over a large area, can beprovided by the base element. As a rule, the carrier pieces are onlyfastened to the structure via the base element. A typical base structureis embodied as a closed ring (“frame”) and the carrier pieces protrudeinwards into the ring; the elastic deformation can then be impressed, inparticular, on ring sections that extend along the direction ofextension of the strain sensor element. Another typical base structureis embodied with a closed surface, on which the carrier pieces areseated.

A preferred development of this embodiment provides for the first andsecond carrier pieces to each be fastened to the base element only in arear region which faces away from the strain sensor element, and for thefirst and second carrier pieces to each be fastened to the strain sensorelement in a front region which faces away from the rear region. Thecarrier pieces are only attached to the structure via the base element,fastening being affected at the rear region of the respective carrierpiece. The carrier pieces typically exhibit no noticeable elasticdeformation (between the front and rear region). As a result of thedistance from the front region to the rear region, the elastic strain ofthe base body or the structure is transferred in reinforced fashion tothe strain sensor element. Typically, the total length GL of the carrierpieces between the front and rear regions is at least three times aslong as the length AE of the strain sensor element between the fixingpoints.

A development in which the part of the carrier which is kept in anelastically deformed state by the structure when the carrier is fastenedto the structure comprises the base element, in particular wherein thebase element is substantially flat in the elastically deformed state, ispreferred. The base element can be embodied in a simple manner with asuitable, elastically relaxed first shape and a desired elasticallydeformed second shape; the carrier pieces and the strain sensor elementcan then be disposed substantially freely on the base element, inparticular also with a desired pretension being impressed on the strainsensor element in the elastically deformed state of the base body. Theelastic bracing of the base element can (possibly also only partially)be used to achieve a bracing (pretension) on the strain sensor element,for instance for setting up the working point. Typically, the baseelement is fastened (for example, glued) to the structure to bemonitored in planar fashion, which is implemented in particularly simplefashion when flat, for example on the side of a railroad track (railweb). Accordingly, in an elastically relaxed state, following thedetachment of the carrier from the structure, the base element istypically arched, in particular in a plane that contains the directionof extension of the strain sensor element. The arching allows theelastic strain of the strain sensor element to be changed in comparisonwith the flat state.

An embodiment that provides for the carrier to comprise at least oneapplication element is particularly preferred, wherein the applicationelement, in the elastically deformed state of at least the part of thecarrier, leaves the strain sensor element or a feed line of the strainsensor element unaffected, and the application element is embodied insuch a way that the application element, in an elastically relaxed stateof at least the part of the carrier following the detachment of thecarrier from the structure, impairs, in particular kinks and/or divides,the strain sensor element or a feed line of the strain sensor element insuch a way that the strain sensor element leaves its working range orcompletely fails. In the elastically deformed state (target state of thestrain sensor element), the application element is typically spacedapart from the strain sensor element (or its feed line, for example asection of an optical fiber), and, in the elastically relaxed state(following the strain sensor falling off), the application element comesinto contact with the strain sensor element (or its feed line). Thestrain sensor element (or its supply line) is typically kinked, squeezedor even divided (for instance cut) by the application element; this canbe easily detected by a measurement, as a result of which the straingauge arrangement falling off can be identified. In this embodiment, thestrain sensor element does not have to adopt any strain components fromthe elastic deformation of at least the part of the carrier, whichfacilitates more precise measurements of the strain of the structure tobe monitored. It should be noted that the change in reflectionwavelength is typically considered when the strain sensor element or FBGis influenced, and damping of the reflected light increases, i.e., thedamping is relevant, in the case of an influence outside the strainsensor element or FBG (effect on feed line).

An embodiment which provides for the strain sensor element to comprisean optical fiber with a fiber Bragg grating, furthermore, a furtherstrain sensor element to be present, wherein the further strain sensorelement likewise comprises this optical fiber and a further fiber Bragggrating, the further strain sensor element to be also fastened to thecarrier, and for, in the elastically deformed state of at least the partof the carrier with the carrier fastened to the structure, the fiberBragg grating to assume a first strain state in which it is transparentin a working range of the further fiber Bragg grating of the furtherstrain sensor element and for, in an elastically relaxed state of atleast the part of the carrier following the detachment of the carrierfrom the structure, the fiber Bragg grating to assume a second strainstate in which the working range of the further fiber Bragg grating ofthe further strain sensor element is non-transparent, or for there to beno overlap of a reflection wavelength interval of the fiber Bragggrating and a further reflection wavelength interval of the furtherfiber Bragg grating between the elastically deformed state of at leastthe part of the carrier with the carrier attached to the structure andan elastically relaxed state of at least the part of the carrierfollowing the detachment of the carrier from the structure is alsoadvantageous. The strain sensor element and the further strain sensorelement can be set up in such a way that the strain sensor element isused solely for the detection of a fall-off and the further strainsensor element is used for the actual detection of the strain of thestructure to be monitored. As a result, the construction, assemblyand/or adjustment of the strain gauge arrangement can be simplified andthe strain measurement of the structure can be more precise. The furtherstrain sensor element need not adopt any strain components from theelastic deformation of at least the part of the carrier. The firstdesign (with a “non-transparent” fiber Bragg grating in the relaxedstate) is comparatively easy to set up, for instance via a strong bendin the fiber Bragg grating or the optical fiber. However, as a rule, thesecond design (with “separated” reflection wavelength and furtherreflection wavelength) allows even more precise strain measurements. Itshould be noted that an elastic pretension of the carrier or of at leasta part of the carrier against the structure is only needed in themonitored area of the strain sensor element (and is preferably only setup in the monitored area of the strain sensor element), but not neededin the monitored area of the further strain sensor element. The furtherstrain sensor element can also be fastened by means of fixing points tothe carrier at mutually spaced apart, opposing carrier pieces.

The scope of the present invention also includes a method for assemblinga strain gauge arrangement according to the invention, as describedabove, which is characterized by the following steps:

a) at least the part of the carrier is elastically braced by means of abracing element;

b) at least the part of the carrier is kept in the elastically bracedstate by means of the bracing element and the carrier is fastened, inparticular adhesively bonded, to the structure to be monitored;

c) the bracing element is removed. This method is particularly simpleand can also, in particular, be used well in an inhospitable environment(e.g. on rails of railroad lines in open countryside). The fasteningaccording to step b) is only complete once the fastening has reached adesired strength (for example after an adhesive has cured). The straingauge arrangement is operated, i.e., the strain of the structure to bemonitored is monitored, only after step c). During operation, there isalso monitoring as to whether the deformation of the carrier stillexists or has been lost as a result of detachment from the structure.

An advantageous variant of the method according to the inventionprovides for the bracing element to be securely connected to the carrierbefore step a) and for the bracing element to be broken off from thecarrier for step a). The bracing element formed on, or fastened to, thecarrier is always available for assembly, and can simply be broken offand used immediately for the bracing process. In particular, the bracingelement can in particular be embodied as a cam.

Likewise, a variant that provides for the bracing element to be securelyconnected to the carrier in steps a) and b) and for the bracing elementto be broken off from the carrier for step c) is advantageous. Here,too, the bracing element, which is initially fastened to, or formed on,the carrier, is always available. Before being broken off, the bracingelement puts or keeps the rest of the carrier under pretension, which isused for the installation. Following the completion of the fastening ofthe carrier to the structure, the bracing element is broken off so thatthe pretension is only kept via the fastening (for example, adhesivebonding or screwing) on the structure, and so the pretension orassociated elastic deformation is lost in the case of a fall-off fromthe structure.

A development of these variants in which, following the assembly of oneor more strain gauge arrangements, the bracing elements which werebroken off and removed in step c) are checked for completeness isadvantageous. This facilitates a simple prima facie check as to whetherthe strain gauge arrangements have been correctly installed.

Further advantages of the invention are apparent from the descriptionand the drawing. Likewise, the aforementioned features and the featuresyet to be explained below can, according to the invention, each find useindividually or together in any combination. The embodiments shown anddescribed should not be understood to be an exhaustive list, but ratherhave an exemplary character for the purpose of illustrating theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated in the drawing and explained in more detailon the basis of exemplary embodiments. In the figures:

FIG. 1a shows a schematic plan view of a first embodiment of a straingauge arrangement according to the invention, with a frame-shaped baseelement;

FIG. 1b shows a schematic cross-sectional view of the strain gaugearrangement of FIG. 1 a;

FIG. 2a shows a schematic plan view of a second embodiment of a straingauge arrangement according to the invention, comprising a leaf springelement, support element and driver element;

FIG. 2b shows a schematic side view of the strain gauge arrangement ofFIG. 2a , comprising a carrier fastened to the structure;

FIG. 2c shows a schematic side view of the strain gauge arrangement ofFIG. 2a , comprising a carrier detached from the structure;

FIG. 3 shows a schematic plan view of a third embodiment of a straingauge arrangement according to the invention, with a bracket;

FIG. 4a shows a schematic plan view of a fourth embodiment of a straingauge arrangement according to the invention, comprising a cam that hasnot yet been broken out;

FIG. 4b shows the strain gauge arrangement of FIG. 4a , with a spreadingcam;

FIG. 4c shows the strain gauge arrangement of FIG. 4b , followingfastening to the structure and with the cam removed;

FIG. 5a shows a schematic side view of a fifth embodiment of a straingauge arrangement according to the invention, comprising a base elementin the elastically relaxed, arched state;

FIG. 5b shows the strain gauge arrangement of FIG. 5a , in theelastically deformed, flat state of the base element;

FIG. 6a shows a schematic embodiment of a sixth embodiment of a straingauge arrangement according to the invention, comprising applicationelements in contact with the strain sensor element;

FIG. 6b shows the strain gauge arrangement of FIG. 6b , with theapplication elements disposed away from the strain sensor element;

FIG. 7 shows a schematic embodiment of a seventh embodiment of a straingauge arrangement according to the invention, comprising a fiber Bragggrating of the strain sensor element and a further fiber Bragg gratingin a further strain sensor element;

FIG. 8a shows a schematic embodiment of an eighth embodiment of a straingauge arrangement according to the invention, comprising a bracket andsecurely connected bow-shaped bracing element, during fastening to astructure;

FIG. 8b shows the strain gauge arrangement of FIG. 8a , followingfastening to the structure, with a broken off bracing element;

FIG. 8c shows the strain gauge arrangement of FIG. 8b , following a fallfrom the structure; and

FIG. 9 shows a schematic view of an axle counter, comprising two straingauge arrangements according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1a shows a first embodiment of a strain gauge arrangement 1according to the invention in a plan view, and FIG. 1b shows anassociated cross-sectional view, cf. plane lb in FIG. 1 a.

The strain gauge arrangement 1 comprises a carrier 2, which herecomprises a substantially rectangular base element 3 that is closed inring-shaped fashion, a first carrier piece T1, and a second carrierpiece T2. The carrier pieces T1, T2 protrude inward from the baseelement 3, which forms an outer frame of the carrier 2.

By way of the frame-shaped base element 3, the carrier 2 is fastened toa structure 4 to be monitored in a manner not shown in detail, e.g., itadhesively bonds to the structure 4 with the entire underside of thebase element 3; however, the undersides of the carrier pieces T1, T2 arenot adhesively bonded to the structure 4, but only rest on saidstructure 4. By way of example, the structure 4 to be monitored can be arailroad track of a train route.

A strain sensor element 5 is fastened to the carrier pieces T1 and T2 atfixing points 7, 8. The fixing points 7, 8 each lie at the inner end ofthe pieces T1, T2. The central section 5 a of the strain sensor element5 located between the fixing points 7, 8 freely spans between the piecesT1, T2 in this case.

In the embodiment shown, the strain sensor element 5 is embodied as anoptical fiber 6 with a fiber Bragg grating (FBG) 6 a in the centralsection 5 a. It should be noted that the fiber 6 is only shown in theregion of the central section 5 a in FIG. 1a for simplificationpurposes; FIG. 1b also shows feed lines 6 b of the fiber 6.Alternatively, other types of strain sensor elements, such as electricalresistance-measuring strips, could also be used.

In a basic state for using the strain sensor element 5 for monitoringthe strain of the structure 4, the strain sensor element 5 is typicallyslightly elastically strained in the direction of extension ER of thefiber 6 in the central section 5 a in order to set the working point ofthe FBG. Here, the elastic strain of the strain sensor element 5 can bepredetermined practically as desired when or by fastening the strainsensor element 5 to the fixing points 7, 8 (in the basic state).

If there is a deformation of the structure 4, the base element 3fastened to the structure 4 is also deformed. This deformation of thebase element 3 is transferred to the strain sensor element 5 via thecarrier pieces T1, T2, as a result of which this deformation becomesmeasurable.

A total length GL of the pieces T1, T2, in each case from the rearregion 9 to the front region 10 where the respective fixing point 7, 8is arranged, with GL=LT1+LT2, is significantly longer than the distanceAE between the fixing points 7, 8 in this case. In the embodiment shown,GL=2*AE applies approximately; in general, GL≥2*AE or GL≥3*AE ispreferred. As a result of the pieces T1, T2 protruding parallel to thedirection of extension ER of the strain sensor element 5, which piecesmove when the structure 4 is deformed along the direction of extent ERbut are themselves hardly deformed, the deformation of the structure 4or the frame in the direction of extent ER can be transferred inreinforced fashion to the strain sensor element 5, in accordance withthe ratio AE/(LT1+AE+LT2).

In the basic state already mentioned, the frame-shaped base element 3 isin an elastically deformed state, in which it is kept by the underlyingstructure 4 to which the base element 3 is fastened. In the presentcase, the base element 3 is strained slightly elastically with respectto the direction of extent ER. Should the base element 3 or the carrier2 fall off the structure 4, the elastic deformation of the base element3 would spring back; i.e., the base element 3 would contract in thedirection of extent ER in this case. This would also change the elasticstrain state of the strain sensor element 5, as a result of which thefall-off from the structure 4 can be uncovered.

In the further described embodiments of strain gauge arrangements, it isprimarily the differences from the embodiment of FIGS. 1a, 1b that areexplained.

A second embodiment of a strain gauge arrangement 1 according to theinvention is illustrated in a plan view in FIG. 2a and, in a side view,in the state fastened to the structure 4 in FIG. 2b and in the detachedstate in FIG. 2 c.

The strain gauge arrangement 1 has a carrier 2, which also comprises aleaf spring element 21 in addition to the pieces T1, T2. In theembodiment shown, the leaf spring element 21 connects the two carrierpieces T1, T2 to one another, the leaf spring element 21 being fastenedwith its ends 22, 23 to the pieces T1, T2 in each case; the end regionsof the leaf spring element 21 extending over the pieces T1, T2.

A support element 24 is formed on the leaf spring element 21, saidsupport element extending over the strain sensor element 5 in this caseand being supported with a support projection 24 a on the structure 4 tobe monitored as a stop partner. When the carrier 2 is fastened to thestructure 4 (cf. FIG. 2b ), the support element 24 presses the leafspring element 21 away from the structure 4 in the upward direction, asa result of which the leaf spring element 21 is elastically deformed,specifically strained in this case.

Moreover, a driver element 25 is formed on the leaf spring element 21,said driver element likewise extending over the strain sensor element 5in this case and having a driver projection 25 a which projects downwardin the direction towards the strain sensor element 5 in this case. Thus,driver projection 25 a engages behind the strain sensor element 5 fromabove. In state where the leaf spring element 21 is elastically deformedby the support on the structure 4 (cf. FIG. 2b ), the driver element 25(or its driver projection 25 a) is raised so far in the embodiment shownthat it does not touch the strain sensor element 5. Accordingly, thestrain sensor element 5 has its normal strain state, which is providedfor normal measurement operation (for strain monitoring of the structure4).

Should the strain gauge arrangement 1 fall off the structure 4, the leafspring element 21 springs back into an elastically non-deformed state,which is shown in FIG. 2c , since the support element 24 no longer has astop partner at the front end. In this state, the driver projection 25 aof the driver element 25 presses on the strain sensor element 5 andimpresses a significant elastic strain thereon. This significant elasticstrain is easy to measure, as a result of which the strain gaugearrangement 1 falling from the structure 4 can easily be detected.

In the embodiment of FIGS. 2a-2c , the carrier pieces T1, T2 or thecarrier 2 can be fastened to the structure 4 to be monitored without anysignificant shear stress, which simplifies the assembly.

FIG. 3 shows a third embodiment of a strain gauge arrangement 1according to the invention in a schematic top view.

In this case, the carrier 2 comprises the pieces T1, T2, to which thestrain sensor element 5 is fastened at the fixing points 7, 8, and abracket 31, which connects the pieces T1, T2 to one another at one end(the lower end in FIG. 3). The direction of extent ER of the strainsensor element ER extends parallel to the bracket 31 or its connectingdirection.

The carrier 2 is fastened to the structure 4 in an elastically deformedstate, specifically with a certain mutual torsion of the pieces T1, T2,approximately around a joint region 32 in the middle of the bracket 31.The torsion acts approximately parallel to the direction of extension ERof the strain sensor element 5 in the region of the strain sensorelement 5.

Should the structure 4 fall off, this torsion springs back, as a resultof which the strain state at the strain sensor element 5 changes. Boththe pieces T1, T2 and the bracket 31 are involved in the torsion.

The distance AS between the bracket 31 (or the upper part thereof) andthe strain sensor element 5 is preferably significantly greater than thedistance AE between the fixing points 7, 8 in order to be able tocontrol the forces in the region of the strain sensor element 5 well. Inthe embodiment shown, approximately AS=3*AE applies; generally AS≥2*AEor AS≥3*AE is preferably chosen.

If desired, the bracket 31 can be weakened in the joint region 32, forexample by way of a notch 33. As a result, larger torsions can beproduced with the same force, for example, which makes it easier todetect a fall-off from the structure. It is likewise possible to disposea spring 34 or a plurality of springs 34 (tension or compressionsprings, as required) between the pieces T1, T2 in order to adjust theelastic behavior of the pieces T1, T2 with respect to one another.

It should be noted that in this embodiment, too, the strain sensorelement 5 can be freely spanned in the central section 5 a, and anelastic strain state of the strain sensor element 5 in the basic statecan, in principle, be freely determined.

FIG. 4a shows a fourth embodiment of a strain gauge arrangement 1according to the invention. In this case, the carrier 2 comprises thecarrier pieces T1, T2, to which the strain sensor element 5 is fastenedwith the fixing points 7, 8, and a bracket 31, which connects thecarrier pieces T1, T2. Prior to the assembly of the strain gaugearrangement 1, a bracing element 41, which is designed as a cam, is keptbetween the carrier pieces T1, T2. The bracing element 41 is typicallymade in one piece with the remainder of the carrier 2 (for example bylaser cutting from a metal sheet) and is held at the remainder of thesupport 2 at two predetermined breaking points 42, between the carrierpieces T1, T2 in this case. In the situation shown in FIG. 4a , thecarrier 2 is in an elastically relaxed state and has not yet beenfastened to a structure. The long side of the bracing element 41 isaligned parallel to the carrier pieces T1, T2 or perpendicular to thedirection of extension ER of the strain sensor element 5. The strainsensor element 5 hangs slack between the carrier pieces T1, T2; thisstate can easily be determined by a strain measurement.

For the assembly on a structure 4 to be monitored, the bracing element41 is broken off from the predetermined breaking points 42, for exampleby turning 43 the bracing element 41 as shown in FIG. 4 a.

As a result of half a rotation of the bracing element 41 or cam, thelatter spreads the pieces T1, T2 apart (or applies torsion with respectto the carrier pieces T1, T2), as a result of which the carrier 2assumes an elastically deformed state, cf. FIG. 4b . A bracing bearing44 for the bracing element 41 is consequently set up between the innersides of the carrier pieces T1, T2; a fixed axis of rotation 45 istypically set up for the bracing element 41. The strain sensor element 5is tightened and receives a slight elastic strain, in accordance withthe working point thereof. The long side of the bracing element 41 isaligned perpendicular to the carrier pieces T1, T2 or parallel to thedirection of extension ER of the strain sensor element 5. In this spreadstate, the carrier 2 is fastened, for example adhesively bonded, to thestructure 4 to be monitored.

After fastening to the structure 4 (for example after an adhesive hascured), the bracing element can be removed, cf. FIG. 4c . For as long asthe strain gauge arrangement 1 remains fastened to the structure 4, theelastically deformed state of the carrier 2 and the slight elasticstrain of the strain sensor element 5 are also retained. Shoulddetachment from the structure 4 occur, the strain gauge arrangement 1would return to the elastically relaxed state shown in FIG. 4a , whichcould easily be measured by the slackening of the strain sensor 5.

In the fifth embodiment of a strain gauge arrangement 1 according to theinvention, shown schematically in a side view in FIG. 5a , the carrier 2has a base element 3, with a closed area in this case, to which thepieces T1, T2 are fastened. In the shown elastically relaxed state ofthe base element 3, in which it is not fastened to a structure 4, thebase element 3 is arched, as a result of which the pieces T1, T2 areclose together and a strain sensor element 5 extending therebetween isslack, which is easy to measure.

In the basic state shown in FIG. 5b , in which the carrier 2 isfastened, for instance adhesively bonded, to the structure 4 to bemonitored (shown in dashed lines) by means of the base element 3, thebase element 3 is kept in an elastically deformed state, a flat state inthis case, and the strain sensor element 5 is tightened so that it canbe used in its working range. In particular, deformations of thestructure 4 along (or with a component along) the direction of extensionER of the strain sensor element 5 can be measured well.

A sixth embodiment of a strain gauge arrangement 1 according to theinvention is illustrated in an elastically relaxed state of a baseelement 3 of a carrier 2 in FIG. 6a and in an elastically deformed stateof said base element 3 in FIG. 6b . Two application elements 61 arearranged or formed on the base element 3.

In an elastically relaxed state of the base element 3, the latterassumes an arched state, as a result of which the application elements61 press on the strain sensor element 5 in a central region 5 a betweenthe carrier pieces T1, T2 (the fastening of which to the rest of thecarrier 2 has not been shown in detail for the sake of simplification),as illustrated in FIG. 6a , or even cut therethrough (not shown indetail). As a result, the strain sensor element 5 is temporarily orpermanently impaired in an easily measurable manner. If the strainsensor element 5 is embodied as an optical fiber with a fiber Bragggrating, the reflection wavelength can be greatly detuned, for exampleby squeezing the fiber, or the attenuation of the fiber can be greatlyincreased.

In an elastically deformed state of the base body 3, in this case, asshown in FIG. 6b , in a flat state of the base body 3 fastened to theflat structure 4 to be monitored, the application elements 61 aredisposed distant from the strain sensor element 5 and do not impair thelatter. Normal strain monitoring of the structure 4 can be carried out.

FIG. 7 shows a seventh embodiment of a strain gauge arrangement 1according to the invention. This includes a strain sensor element 5 anda further strain sensor element 71 in this case. The strain sensorelement 5 is formed with a fiber Bragg grating 6 a (also called a slaveFBG or auxiliary FBG) (cf., e.g., FIG. 1a above in this respect), andthe further strain sensor element 71 is formed with a further fiberBragg grating 72 (also called master FBG or main FBG). The fiber Bragggrating 6 a and the further fiber Bragg grating 72 are formed atdifferent positions in the same optical fiber 6.

In the case of the strain sensor element 5, the two carrier pieces T1,T2 are fastened via the bracket 31 to the structure 4 in an elasticallydeformed state in relation to the structure 4 to be monitored (cf.,e.g., FIG. 3 above); to this end, an elastic tension was typicallyapplied locally to the carrier 2 during fastening, for instance using acam or another bracing element (not illustrated in more detail). Shouldthe elastic deformation of the carrier 2 be lost in the monitored regionof the strain sensor element 5, this would be recognized by asignificant change in the strain state on the strain sensor element 5.

At the further strain sensor element 71, the optical fiber 6 is fastenedto carrier pieces T1′, T2′ with fixing points 74, 75 in this case, withthe optical fiber 6 freely spanning between the fixing points 74, 75.The carrier pieces T1′, T2′ are also in this case interconnected by wayof a bracket 73, but there is no local elastic bracing or deformation ofthe carrier 2 with respect to the structure 4. Thus, in the monitoredregion of the further strain sensor element 71, the carrier 2 transfersa deformation of the structure 4 without a tension offset by the localcarrier 2.

In the embodiment shown, the fiber Bragg grating 6 a and the furtherfiber Bragg grating 72 have significantly different, non-overlappingreflection wavelength intervals. By way of example, the reflectionwavelength of the fiber Bragg grating 6 a (primarily depending on thedeformation state of the local carrier 2) can be in the range of 720 nmto 670 nm, and the further reflection wavelength of the further fiberBragg grating 72 (primarily depending on the strain state of thestructure 4) can be in the range of 635 nm to 625 nm. Then, the correctfit on the structure 4 can be checked on the basis of the currentreflection wavelength of the strain sensor element 5, and thedeformation of the structure 4 can be checked on the basis of thecurrent, further reflection wavelength of the further strain sensorelement 71; both are measurable at the same time. Alternatively, thestrain sensor element 5 or the strain gauge arrangement 1 could be setup in such a way that, in the event of falling off, the fiber Bragggrating 6 a becomes non-transparent in the working range (correspondingto the entire further reflection wavelength interval) of the furtherfiber Bragg grating 72, which can easily be uncovered when an attempt ismade to determine the reflection wavelength of the further fiber Bragggrating 72.

Typically, the local parts of the carrier 2 are fastened in the same wayin the monitored region of the strain sensor element 5 and in themonitored region of the further strain sensor element 71, and so anequal hold (for example as a result of aging) can generally be expected.Preferably, all parts of the carrier 2 are mechanically coupled to oneanother, in a manner not shown in detail, so that all parts of thecarrier 2 behave in the same way (in particular all are fastened to thestructure 4 or all fall-off therefrom).

FIG. 8a shows an eighth embodiment of a strain gauge arrangement 1according to the invention. The carrier 2 comprises the pieces T1, T2,to which the strain sensor element 5 is fastened at the fixing points 7,8. Firstly, the pieces T1, T2 are securely connected to one another viaa bracket 31 of the carrier 2, and, secondly, also securely connected toone another via a bow-shaped bracing element 81 in this case.

The bracing element 81 forces an elastic deformation in the bracket 31,with the pieces T1, T2 being spread apart from one another in this case(or being twisted against one another, cf. the description of FIG. 3above in the respect as well). In this state, the carrier 2 (with thepieces T1, T2 and the bracket 31 but not with the bracing element 81) isfastened, for example adhesively bonded, to a structure 4 to bemonitored.

After complete fastening, the bracing element 81 is removed; to thisend, the bracing element 81 is broken off the carrier 2 at predeterminedbreaking points 82, cf. FIG. 8b . The elastically spread state of thebracket 31 is maintained by the structure 4 to which the carrier 2 isfastened. The regions of the fracture surfaces remaining on the carrierpieces T1, T2 can be considered to be a bracing bearing 83 for thebracing element 81.

Should the strain gauge arrangement 1 fall off the structure 4, thebracket 31 subsequently assumes an elastically relaxed state. This isshown in FIG. 8c . As a result, the pieces T1, T2 approach each otherand the strain sensor element 5 slackens. This is easy to measure on thestrain sensor element.

FIG. 9 illustrates in a schematic side view an axle counter 91 for theinvention, which is fastened laterally to a railroad rail 92 (forinstance to the so-called rail web) as structure 4 to be monitored. Theaxle counter 91 in this case comprises two strain gauge arrangements 1according to the invention, each of which is formed in exemplary fashionin this case with two pieces T1, T2 and a bracket 31 connecting them(cf., e.g., FIG. 3). Trains whose train axles can be detected by theaxle counter 91 can travel on the rail 92 in the direction of travel FR.The directions of extension ER of the strain sensor elements 5 runobliquely to the direction of travel FR, at an angle of approximately45° in the embodiment shown. It should be noted that the direction oftravel FR corresponds at the same time to the direction of a neutralfiber in the rail 92 or structure 4 under the load of the trains.

If, for example, a train axle rolls from left to right in FIG. 9 abovethe axle counter 91, the left strain gauge arrangement 1 first registersa relative strain and then a relative compression in relation to a basicstate (without a train crossing). Somewhat later, the right strain gaugearrangement 1 also registers a relative strain first, and then arelative compression in relation to the basic state. In the case of anopposite direction of travel, the sequence of events is reversedaccordingly.

A redundancy can be set up as a result of the two strain gaugearrangements 1 in the axle counter 91, improving safety in trainoperation. It should be noted that, in simpler embodiments, only onestrain measuring arrangement may be provided in the axle counter 91.

LIST OF REFERENCE SIGNS

-   -   1 Strain gauge arrangement    -   2 Carrier    -   3 Base element    -   4 Structure to be monitored    -   5 Strain sensor element    -   5 a Central section    -   6 Optical fiber    -   6 a Fiber Bragg grating    -   6 b Feed line    -   7 First fixing point    -   8 Second fixing point    -   9 Rear region    -   10 Front region    -   21 Leaf spring element    -   22 End (leaf spring element)    -   23 End (leaf spring element)    -   24 Support element    -   24 a Support projection    -   25 Driver element    -   25 a Driver projection    -   31 Bracket    -   32 Joint region    -   33 Notch    -   34 Spring    -   41 Bracing element (cam)    -   42 Predetermined breaking point    -   43 Direction of rotation    -   44 Bracing bearing    -   45 Fixed axis of rotation    -   61 Application element    -   71 Further strain sensor element    -   72 Further fiber Bragg grating    -   73 Further bracket    -   74 Fixing point    -   75 Fixing point    -   81 Bracing element (bow-shaped)    -   82 Predetermined breaking point    -   83 Bracing bearing (for bow-shaped bracing element)    -   91 Axle counter    -   92 Railroad track    -   AE Fixing point spacing    -   ER Direction of extension (strain sensor element)    -   GL Total length of pieces T1, T2    -   FBG Fiber Bragg grating    -   FR Direction of travel    -   LT1 Length of piece T1    -   LT2 Length of piece T2    -   T1 First piece (strain sensor element)    -   T1′ First piece (further strain sensor element)    -   T2 Second piece (strain sensor element)    -   T2′ Second piece (further strain sensor element)

What is claimed is:
 1. A method for assembling a strain gaugearrangement for an axle counter, the method comprising the steps of: a)providing at least one strain sensor element; b) providing a carrier, towhich the strain sensor element is fastened; c) providing a structure tobe monitored, being a railroad track, to which the carrier is fastened,wherein at least a part of the carrier is kept in an elasticallydeformed state by the structure when the carrier is fastened to thestructure; d) embodying the carrier with a first carrier piece and asecond carrier piece, which oppose one another in spaced apart fashion;e) fastening the strain sensor element to the first carrier piece withat least one first fixing point, to the second carrier piece with atleast one second fixing point, and to neither the first nor the secondcarrier piece with a central section between the fixing points; f)elastically bracing at least the part of the carrier by means of abracing element; g) keeping at least the part of the carrier in theelastically braced state by means of the bracing element and fasteningthe carrier to the structure to be monitored; and h) removing thebracing element.
 2. The method as claimed in claim 1, wherein thebracing element is securely connected to the carrier before step f) andwherein the bracing element is broken off from the carrier for step f).3. The method as claimed in claim 2, wherein following the assembly ofone or more strain gauge arrangements, the bracing element which werebroken off and removed in step h) are checked for completeness.
 4. Themethod as claimed in claim 1, characterized in that the bracing elementis securely connected to the carrier in steps f) and g) and wherein thebracing element is broken off from the carrier for step h).
 5. Themethod as claimed in claim 4, wherein following the assembly of one ormore strain gauge arrangements, the bracing element which were brokenoff and removed in step h) are checked for completeness.
 6. The methodas claimed in claim 1, including the step of using of the strain gaugearrangement to count axles.
 7. The method as claimed in claim 1,wherein, when the part of the carrier which is kept in the elasticallydeformed state by the structure when the carrier is fastened to thestructure comprises the first carrier piece and the second carrierpiece, the elastically bracing at least of the part of the carrierincludes elastically bracing the first carrier piece and the secondcarrier piece with respect to one another with at least a directioncomponent along an extension direction of the strain sensor element. 8.The method as claimed in claim 7, wherein the first and second carrierpiece are connected to one another by a bracket, wherein the bracket isspaced apart from the strain sensor element by a distance AS, where thefollowing applies to the distance AS and a distance AE from the first tothe second fixing point: AS≥1*AE.
 9. The method as claimed in claim 8,wherein the carrier forms a bracing bearing, at or in which the bracingelement, which engages on the first and second carrier piece, isarrangeable.
 10. The method as claimed in claim 9, wherein duringassembly the bracing element arranged at or in the bracing bearing isbroken off from the carrier from the inner region of the bracingbearing.
 11. The method according to claim 9, wherein the bracingbearing and bracing element interact in the style of a cam.
 12. Themethod as claimed in claim 7, wherein the first and second carrier pieceare connected to one another by a bracket, wherein the bracket is spacedapart from the strain sensor element by a distance AS, where thefollowing applies to the distance AS and a distance AE from the first tothe second fixing point: AS≥3*AE.
 13. The method as claimed in claim 12,wherein the carrier forms a bracing bearing, at or in which the bracingelement, which engages on the first and second carrier piece, isarrangeable.
 14. The method as claimed in claim 13, wherein duringassembly the bracing element arranged at or in the bracing bearing isbroken off from the carrier from the inner region of the bracingbearing.
 15. The method according to claim 13, wherein the bracingbearing and bracing element interact in the style of a cam.
 16. Themethod as claimed in claim 7, wherein the carrier forms a bracingbearing, at or in which the bracing element, which engages on the firstand second carrier piece, is arrangeable.
 17. The method as claimed inclaim 16, wherein during assembly the bracing element arranged at or inthe bracing bearing is broken off from the carrier from the inner regionof the bracing bearing.
 18. The method according to claim 16, whereinthe bracing bearing and bracing element interact in the style of a cam.19. The method as claimed in claim 1, wherein in step a), the at leastone strain sensor element is chosen as an optical fiber with a fiberBragg grating.
 20. The method as claimed in claim 1, wherein in step g),fastening the carrier to the structure to be monitored is done byadhesively bonding the carrier to the structure to be monitored.